The present invention relates in general to a microwave apparatus for generating plasma afterglows, and more particularly, to such an apparatus including a chamber for plasma afterglow stripping and/or etching of photoresist and selective isotropic etching of semiconductor material such as polysilicon and silicon nitride using a variety of etching compositions which form reactive species in a plasma. Still more particular, the present invention further relates to such an apparatus for anisotropic etching of semiconductor material in a chamber employing a downstream plasma afterglow which is subjected in situ to radio frequency (RF) power.
Plasma stripping and etching are generally well-known techniques used in the fabrication of semiconductor devices and integrated circuits, wherein a gas composition which has been disociated into radicals and positive and negative ions react with unprotected areas of, for example, photoresist and semiconductor material. These techniques have achieved wide acceptance in the semiconductor industry because they are dry and exceptionally clean, eliminate processing and handling steps previously necessary in chemical etching, and use only small quantities of safe and inexpensive gases, thus avoiding the necessity for storage and use of large quantities of expensive noxious and difficult to dispose of solvents and acids. Moreover, plasma etching techniques offer a high potential of resolution capability and reliability for high density processing over conventional known wet chemical processing techniques.
One such plasma etching technique is known from U.S. Pat. No. 4,065,269, which generally discloses plasma etching using a microwave afterglow. In this technique, the plasma is created outside the etching chamber in an upstream microwave reactor with the reactive species, i.e., microwave afterglow, being transported to the chamber where plasma etching takes place. Etching using a microwave afterglow has been reported as being used for stripping the remaining photoresist from a substrate, as well as to selectively remove semiconductor material such as polysilicon and silicon nitride by isotropic etching through a mask. Although plasma etching using a microwave afterglow is generally known, the disclosed microwave reactor for generating the reactive species and etching chamber where the plasma etching takes place, suffer from a number of disadvantages. For example, by employing the microwave reactor substantially upstream of the etching chamber, frequent collision of the reactive species while being transported to the etching chamber results in their recombination into inactive species thereby adversely affecting the rate of etching. In addition, the known microwave reactor is relatively complex in construction and relatively inefficient, vis-a-vis microwave coupling to generate a plasma, in providing a continuous source of reactive species to the etching chamber. Overall, the known microwave reactor and etching chamber, although apparently suitable for laboratory use, is constructed in a manner which renders it unsuitable for scale-up and use in commercial plasma etching as required in the highly automated semiconductor industry.
In the manufacture of semiconductor devices and integrated circuits, there is the necessity of selectively modifying the electrical and physical properties of the substrates from which the devices and circuits are to be made. In recent years, there has been an ever increasing trend towards large scale integration in the manufacture of such diverse devices as 16-bit microprocessors, 256K and up memory chips, etc. Because of the extremely high component density in such devices, the line widths which are required to fabricate and interconnect these devices on semiconductor chips approach submicron dimensions. Isotropic etching tends to undercut the photoresist at the photoresist-semiconductor interface as a result of isotropic etching characteristically having etch rate which is the same in all directions. The practical effect of this undercutting is that line widths formed in these device are substantially narrower than those which were contained in the overlying photoresist mask.
One solution to this problem is the use of anisotropic etching as generally known from U.S. Pat. No. 4,253,907. Anisotropic etching requires the use of an etching chamber having the semiconductor wafer supported by an electrode which is connected to an RF power source. The RF voltage dropped between the plasma and the electrode results in a large DC bias which accelerates positive ions towards the electrode supported wafer, resulting in anisotropic etching. There is also known from U.S. No. 4,282,267 an apparatus for generating a plasma suitable for anisotropic etching using a combination of reactive species. The reactive species, each having different activation levels, are separately subjected to microwave and RF power sources within a common batch type plasma reactor. However, this known plasma reactor also suffers from a number of disadvantages as well. For example, this plasma reactor, by being a batch processor, does not lend itself readily for integration in a highly automated semiconductor devices manufacturing process. As a batch type plasma reactor, there further requires extensive manual handling of the wafers which can result in their contamination and/or damage. Furthermore, because of the large number of wafers contained in each batch load, etching is often not very uniform between wafers positioned at various locations within the reactor. This is a severe disadvantage when anisotropic etching is employed to manufacture semiconductor devices which require line widths having submicron dimensions.